Demodulation of heterodyne interferometric signals has traditionally been accomplished with a combination of hardware and firmware as shown in FIG. 1. The front end is comprised of various analog circuit stages. This includes a photodiode 5, a trans-impedance amplifier (TIA) 10, analog frequency mixers (20, 22), and low-pass filters (30, 32). The signal outputs of the LPFs are digitized by analog to digital converters (40, 42). The two digital values (the In-phase and Quadrature components of the incoming carrier signal) are then used to calculate an arctangent value and recover the phase signal of interest. This arctangent computation is performed in the digital domain and can be accomplished through the use of either a digital signal processor (DSP) or Field Programmable Gate Array (FPGA). However, calculation of the arctangent value in the digital domain is very computationally intensive. Likewise, an exponential relationship exists in the required computational resources used and the error in the computed valued. The lower the required error, the more computational resources and processing that is needed.
Various techniques exist for computing the arctangent within a digital system. The three most common methods include look-up-tables (LUTs), the CORDIC algorithm, and a Taylor series computation. All three methods however, would require an excessive amount of computational cycles to meet the low error requirement (<luradian) of a low phase noise heterodyne demodulator.
As opposed to the analogue to digital demodulator outlined above and shown in FIG. 1, the invention is directed towards an all-digital demodulator. It should be noted that some all-digital demodulator designs are also known in the art. For example, FIG. 2 is an exemplary illustration of an all-digital demodulator that is known in the prior art. As compared to the arrangement of FIG. 1, in the embodiment of FIG. 2, the analog frequency mixers and the analog LPFs have been moved to the digital domain 52. A single ADC 41 now directly digitizes the incoming heterodyne carrier signal 10, following the photodiode 5 and TIA 10. The I/Q frequency mixing is now performed using digital multipliers (21, 23). Finite Impulse Response (FIR) filters (31, 33) are used to replace the analog low-pass filters. The FIG. 2 illustration is a comparable digital implementation of the above analog embodiment, however, there is no inherent reduction in arctangent processing requirements using the all-digital approach. It is desired to have a digital I/Q reprocessing demodulator that improves upon the above-outlined basic digital demodulator, particularly one that is faster and utilizes processing time more efficiently.